UPDATED Redding weather: Snow levels drop to Lake Shasta this evening

Showers likely in the mountains, possible in the valley on Wednesday

Flash Flood Watch: The National Weather Service has extended a Flash Flood Watch for the burn scars in Shasta and Tehama counties through 10 p.m. today. Heavy rains could trigger debris flows over roads.

Winter Storm Warning: The National Weather Service has posted a Winter Storm Warning for the southern Cascades through 10 a.m. Wednesday for 1 to 2 feet of snow above 5,000 feet and wind gusts up to 45 mph.

Lake Shasta below the dam

These figures compare the distance of Lake Shasta's surface from the dam crest to the day before and last year on this date. We also get this season's Oct. 31 low point and how much the lake has risen since that time. Information courtesy of the U.S. Bureau of Reclamation. Figures are in feet:

• Current: 101.24

• Yesterday: 107.1

• Last year: 73.22

• Low point: 157.77

• Above low point: 56.53

Lake Shasta storage

These figures compare current storage in Lake Shasta with storage at approximately the same date during prior drought periods. Percentage change noted in lake level is 24 hours. Information courtesy of the U.S. Bureau of Reclamation. Totals are in acre-feet:

• Current: 2,148,812 (47.20 percent full, up 2.28 percent)

• Last year: 2,695,389 (59.21 percent full)

• 1992: 2,001,348 (43.96 percent full)

• 1991: 1,596,112 (35.06 percent full)

• 1977*: 1,460,700 (32.08 percent full)

• Capacity: 4,552,000

*Only monthly figures available so comparison date is March 1.

Fab Feb, Miracle March

We've seen it before. Deluge in the midst of drought that dumps loads of badly-needed water into Lake Shasta -- though never enough (at least not yet) to fill the lake. These figures compare the recent "Fabulous February" (can we hope for a "Marvelous March?") with wet months during the 1986-1992 drought. Precipitation is inches, lake inflow is acre-feet, lake water level changes and lake elevations are feet above sea level:

March 2009*

• Shasta Dam precip: 3.40

• Risen this month: 10.77 ft.

• Current elevation: 965.76

• Low water this past fall: 909.23

February 2009

• Shasta Dam precip: 18.76

• Rise that month: 35.62 ft.

• Inflow that month: 663,873

March 1989

• Shasta Dam precip: 14.92

• Rise that month: 69.54 ft.

• Inflow that month: 1,501,800

• High water that spring: 1,038.15

• Low water that fall: 959.40

March 1991

• Shasta Dam precip: 22.26

• Rise that month: 29 ft.

• Inflow that month: 607,490

• High water that spring: 968.77

• Low water that fall: 912.11

February 1992

• Shasta Dam precip: 19.72

• Rise that month: 42.38 ft.

• Inflow that month: 761,000

• High water that spring: 992.62

• Low water that fall: 935.38

*As of today

Airport rainfall to date

Here are precipitation totals for Redding Municipal Airport and season norms to date, as of 5:30 a.m. today. Information courtesy of the National Weather Service. Totals are in inches:

• Last 24 hours: 0.03

• March total: 0.78

• March normal: 0.38

• Season to date: 18.14

• Season normal to date: 24.00

• Percent of normal to date: 75.58

• Last year to date: 22.92

UPDATE 7:48 p.m.

While rain poured down at nearly a half-inch an hour near Lakehead, temperatures and dew points steadily dropped from the mid-40s early this afternoon to near freezing this evening.

The heavy precip has ended around Lakehead, but what's falling now is coming down as snow, prompting the California Department of Transportation to post chain requirements on I-5 from near Dunsmuir up through Weed.

Colder air behind the shortwave that moved through this evening has dropped temperatures throughout the north state.

Redding Muni went from 52 to 45 degrees in just a half-hour between 5:20 and 5:50 p.m. this evening while winds shifted from the southeast to the west-southwest, signaling some kind of frontal passage.

Heavy rain and thunderstorms moved through the Redding area around this time, suggesting the showers were sparked by this wave.

The central low pressure core off the Oregon border spawning all this weather will keep spinning away over the ocean Wednesday while drifting south and filling in.

So we will see the cool temperatures and shower threat continue Wednesday. But from the look of things, any showers that develop won't be as intense as they were today, and shower chances will diminish as the day goes on.

Ditto the wind as the offshore low fills. Pressure gradients will ease and winds will likely wane.

Afternoon temps will likely top out in the unseasonably cool upper-mid 50s as cold air continues pooling over the north state as the polar low drifts south.

Robust rain is falling at the R-S office on the north side of town. We've seen lightning and heard thunder.

These cells seem to be moving rapidly, unlike last Monday's training cells. Rain should not last too long, but will monitor.

Meanwhile, a station near Lakehead reported 0.48 inches between 4 and 5 p.m. as heavy rain continues along the I-5 corridor north of town. Those rain rates, while impressive, are still only a third of those recorded during last week's training cell event.

Heavy rain is imminent in west Redding and will be spreading rapidly northeast toward Enterprise, Bella Vista, etc.

UPDATE 5:11 p.m.

Radar indicates very heavy rain falling on the north slope of Shasta Bally above Whiskeytown Lake, an area burned in this summer's wildfires. But the heaviest rain is rapidly moving northeast.

The cells are continuing to train up toward Lake Shasta, where Lakeshore reported 0.40 inches of rain between 3 and 4 p.m. While that's officially heavy rainfall, it pales beside the 1.55-inch hourly rates recorded near Lakehead last Monday, when flooding closed Interstate 5 near the Antlers Bridge.

Radar also indicates light to moderate showers pushing out of the foothills between Redding and Anderson. These showers are heading northeast.

UPDATE 3:44 p.m.

Radar indicates the narrow band of showers is intensifying from about Ono up through Whiskeytown Lake. That means heavy rain could be falling over the burn scars of the Moon Complex, the Whiskeytown fires and the southern Motion Fire.

The gauge at Oak Bottom at Whiskeytown is not picking up heavy precipitation yet, however.

Radar indicates the cells staying stationary over the mountain front. If anything, strong southeast winds are driving the heaviest echoes away from the valley.

UPDATE 2:07 p.m.

Radar shows a line of moderate to borderline heavy showers stretching from near Ono through Whiskeytown, Iron Mountain, Shasta Dam and up the Sacramento River arm of Lake Shasta -- precisely the areas burned in last summer's wildfires.

Satellite shows a bright band of convective clouds exploding over western Shasta County -- one of the signatures of heavy showers.

Finally, a look out the window confirms a wall of blue-black clouds up toward Shasta Dam, while brisk southeast winds are funneling up the valley, , feeding the convergence that's creating these showers.

We saw a similar line of showers develop yesterday, but then weaken and move off to the north without doing much. Will monitor these showers to see if they persist, strengthen and start to drop heavy rains over the burn scars.

END UPDATE

Today will be another mixed bag -- cool, breezy, mostly cloudy (but with bursts of sunshine) and sometimes, perhaps, showery. Maybe even a thunderstorm, but probably not.

"Unsettled," in a word.

Readers may think the term "unsettled" a cop out, that forecasters use it when they don't know what the weather will be. And that's true.

Yet the term is appropriate when you've got an atmosphere so turbulent conditions will literally change from one minute to the next and vary widely from one place to the next.

What we've got going on

All that talk earlier about subtropical jets, Hawaii, swamping rains, pineapples -- out the window.

Now we're on the cold side of the cyclone. In fact, over the last 12 hours or so, a cold 995-millibar low has formed off the Oregon border and is swinging moisture our way while mixing it up with polar air continuing to funnel down the back side of the long-wave trough that's allowed all this wet weather of late.

And surface charts show a cold front draped from this low and pushing northeast into Northern California.

So here's what we can expect (maybe) -- temperatures topping out in the 51-to-55 degree range and up-valley winds hitting sustained speeds up to 20 mph with gusts approaching 30 mph. These winds will probably die off overnight.

As for showers, chances are better in the foothills and mountains, but bursts of rain could hit anywhere.

It looks like our best chances for more showers will be later this morning into early afternoon as this front, or wave approaches.

Showers could crank up again late tonight or early Wednesday morning as this low pinwheels another wave through.

Snow levels will start out around 3,500 feet but could drop to 2,500 feet by evening and locally lower where a thunderstorm develops, should that happen.

Cold air makes thunderstorms boil

All that cold air means an unstable atmosphere -- air cooling rapidly with height. And forecast soundings continue to show a slightly unstable atmosphere over Redding today.

So any air column rising from sun-warmed ground would likely stay warmer than its environment and keep rising, forming convective columns and possible thunderstorms.

Thunderstorms certainly popped in this environment on Monday -- but down in the Sierra, from Butte County through Tuolumne County and out into the San Joaquin Valley.

With a fairly deep low off to our northwest and strong cyclonic flow overhead (counter-clockwise flow, up from the southwest), this space will continue to monitor for training cell thunderstorms forming over the western foothills.

What we've had going on

Showers strafed the north state overnight, with many spots picking up a dime or two. The big dollar totals continued to focus on the mountains around Lake Shasta, where McCloud Dam sopped up 1.48 inches and Slate Creek, 1.40 inches between 4 p.m. Monday and 4 a.m. today.

Of course, all that rain and melted snow is boosting lake levels, where are up 12 feet since Sunday alone. The lake was 101.65 feet from the crest early this morning and rising.

Heavy precip also kept cranking around Mt. Shasta, while the town of Mount Shasta itself soaked up another 0.65 inches from 4 p.m. Monday to 4 a.m. today. The storm total in Mount Shasta since Sunday had swollen to 5.68 inches as of 4 a.m.

Some of that precip is finally counting as snow for the ski park, which reports 16 inches of new powder atop the mountain.

Interesting weather winding down

The low off the Oregon border is forecast to weaken (fill) and drift south Wednesday. That means at least one more day of unsettled weather, though Wednesday probably won't be nearly as unsettled as today.

By Thursday, the low drifts well southwest of us and our weather should start to really settle down -- though not if the low stays closer to the coast than expected.

Mid-range forecasts continue to suggest a fundamental rearrangement of the hemispheric pattern after Thursday.

The long-wave troughing that allowed Alaskan air to pool off the west coast, mix with warmer air and send storms into California slides east, replaced by a long-rage ridge that would send storms up and over California.

Next week looks mostly dry, in other words.

We'd sit on the very edge of this ridging pattern, under a rather cool northwest flow.

And there's some indication polar lows sliding down the west coast may try to back into the ridge, nudging it west and carving out a pocket over the Pacific to give us at least a shot at some showers Friday and again Monday.

But chances are better the ridge will hold, the polar lows will stay to our northeast and we'd come under a dry, cool, somewhat breezy pattern.

Long-range consensus does suggest some possibility that another long-wave trough may form off the west coast after mid-month, perhaps bringing another round of wet weather to California.